I have tried two different fully charged batteries and two different sd cards-including a new one. After powering on everything seems fine until I press the shutter release then it just totally shuts down. Ideas?

SOURCE: shutter release button problum

U have this problem and contemplating whether the price of repair will be worth it. But anyway I was doing a shoot today and I couldn't press anything out of it. I took out my cable release and I got through it.

SOURCE: Canon 40d-pressing the shutter seems to short out the camera.

I recommend that you remove both batteries for ten minutes to get a full reset. you spoke of the main battery. When you open the battery compartment, look at the lower right toward the camera with the LCD facing you.
It is a little hard to see, but will say CR2016 3V. Stick your fingernail under the lip and just pull it out.
As I said I left both batteries out for about 10 minutes.
You will have to re-set the date and time after your done.

SOURCE: 40D turns off as I press shutter button

The shutter mechanism had to be replaced. I've heard that this is a typical 40D problem, due to a shutter design flaw. They say that it has been fixed on replacement shutters.